This invention relates to a method for removal of particles from a gas stream by electrostatic charging and separation of the particles. More particularly, the invention relates to a method for controlling or even preventing the onset of back corona in a high intensity ionization system for electrostatic charging high resistivity-type particles in a gas stream.
A high intensity ionizer is primarily used as a precharger for an electrostatic precipitator. It also finds use, however, as a precharger for a variety of other collection devices including inter alia, fabric bags, Venturi scrubbers and fixed and fluidized bed collectors. Back corona undesirably lowers the particle charging potential of the high intensity ionizer. The onset of back corona is caused by the undesired deposition of high resistivity dust particles on the corona collecting electrode or anode of the high intensity ionizer device.
It has long been recognized in the electrostatic precipitator art that collection efficiency is significantly influenced by the resistivity of the collected particulates. A system for collecting particles having a high resistivity must typically be provided with excess collection area to account for the problem of back corona. High resistive particulates are present in a variety of waste streams, the most prominent being the emissions from a coal-fired boiler employing a low sulfur coal.
The aforementioned layer of high resistivity dust particles represents a resistance to the current which must flow from the discharge electrode to the collector electrode. As a result of this resistance, a voltage gradient develops across the dust layer. The magnitude of this voltage gradient is determined by two factors: the resistivity of the dust and the current density between the discharge and collection electrodes. The dust layer can only withstand a certain voltage gradient. If the voltage gradient increases above this threshold value, a corona flask occurs across the dust layer. This arc produces a large quantity of ions, most of which have a polarity opposite to the particles charged by the discharge electrode. Since the oppositely charged ions cause a net reduction in the overall charge on the entrained particulates, the presence of back corona tends to generally reduce the effective charging level of the particulates. A reduction of collection efficiency ensues.
As noted, the magnitude of the voltage gradient is determined by the resistivity of the dust and the current density between the discharge and collecting electrode. Since the collecting electrode must operate with a layer of dust in order to satisfy its collection function and since the current density must be maintained at a high level to ensure efficient charging, it has long been recognized in the electrostatic precipitator art that a way of reducing resistivity of the dust must be the primary solution to the problem of back corona. Moreover, since the voltage gradient also influences the degree to which the dust is retained by the collecting electrode and accordingly influences the force necessary to remove the dust therefrom, a reduction in the dust resistivity should also benefit collection efficiencies. Therefore, the electrostatic precipitator art has taught a variety of chemical conditioning agents which can be used to reduce resistivity.
As taught by the art, the primary conditioning agent is moisture. Moisture is added to the system by humidifying the particulate laden gas stream. Other conditioning agents considered useful by the prior art, such as sulfur trioxide and ammonia, act as secondary conditioning agents by increasing the water adsorption characteristics of the dust. Moisture conditioning has been effected by direct steam addition, by water spray or by the direct wetting of the raw materials used in the industrial process itself. It has been recognized in the electrostatic precipitator art that at ambient temperature most particulates may be effectively conditioned by only 1 to 2% moisture in the gas; 10 to 20% moisture is commonly needed at 250.degree. F. to 300.degree. F.
Schwab et al U.S. Pat. Nos. 4,093,430 and 4,110,806 describe a recent technological advancement in air pollution control, in particular the removal of fine particles of 0.1 .mu.m to 3.0 .mu.m diameter. These patents describe a high intensity ionization system (hereafter referred to as "HII system" or "HII device") wherein a disc-shaped discharge electrode is inserted in the throat of a Venturi diffuser. A high D.C. voltage is imposed between the discharge electrode or cathode and the Venturi diffuser, a portion of which acts as an anode. The high voltage between the two electrodes and the particular construction of the cathode disc produces a stable corona discharge therebetween of a very high intensity. Particles in the gas which pass through the electrode gap of the Venturi diffuser are charged to very high levels in proportion to their sizes. The entrained particulates are field charged by the strong applied field and by ion impaction in the region of corona discharge between the two electrodes. The high velocity of the gas stream through the Venturi throat prevents the accumulation of space charge within the corona field established at the electrode gap, and thereby improves the stability of the corona discharge between the two electrodes.
In its principal use, the HII system is used as a precharger for an electrostatic precipitator assembly. In this design, the entire assembly is functionally analogous to a conventional two-stage electrostatic precipitator. The HII system, however, operates as a much more effective precharger than the ionizer stage of the conventional two-stage precipitator. A plurality of individual HII devices are aligned with their respective axis parallel to one another, to present a honeycomb-like array of flow passages to the particulate laden feed gas stream. The discharge end of the HII array is then aligned directly in front of an electrostatic precipitator unit.
While the HII device has been shown to be an effective precharger for conventional electrostatic precipitator units, operation has shown that in many cases the phenomenon of back corona impairs the overall charging efficiency of the device. It has been observed that when a high resistivity dust is to be charged by the high intensity ionizer, the dust tends to collect on the anode portion of the Venturi diffuser. Since the anode of the high intensity ionizer is not designed to be a collector electrode, this layer of high resistivity particles causes considerable problems. Moreover, since the current density in a high intensity ionizer is much higher than in a conventional electrostatic precipitator, the back corona problem created by the deposition of high resistivity particles on the anode can be expected to be much more intense.
Satterthwaite U.S. Pat. No. 4,108,615 discloses an HII system which reduces the problems caused by the collection of high resistivity dust on the anode. In this improved HII, the portion of the Venturi wall serving as the anode is formed with a series of axially spaced conical vanes. The vanes are shaped to direct jets of clean purge air along the anode wall is essentially the same direction as the main gas stream. According to this patent the purge gas layer forms an effective barrier to the deposition of particulate matter on the anode and also serves to scrub the anode of any particulates that may collect thereon. However, it appears that very high purge gas velocities through the vanes are required to provide the required cleaning effect. For example, in the known practice of the Satterthwaite improvement, the flow rate of the purge gas is at least about 6% of the main feed gas flow rate through the ionizer, and in many instances approaches 20%. However, in many cases in which a gas containing a high resistivity dust is being treated, this very high purge gas velocity does not provide the necessary cleaning of the HII system. In addition to requiring high purge gas flow rates to clean the anode of deposited particulate matter, the prior art has also taught that the purge gas must first be preheated. This preheating was believed necessary to avoid corrosion of the outlet cone of the HII device caused by the formation of sulfuric acid thereon. The prior art believed that the use of an ambient temperature purge gas tends to cool the outlet cone of the HII device allowing water in the main gas stream to condense and collect thereon. The condensed water on the outlet cone combines with sulfur trioxide in the exhaust emissions from a typical coal-fired boiler and forms sulfuric acid, which accordingly corrodes the outlet cone. However, as will be discussed hereafter, it has been discovered that such preheating only worsens the problem of back corona.
An object of this invention is to provide an improved high intensity ionization system of the purge gas-vaned anode type for separation of high resistivity particles from a gas stream.
Another object is to provide an improved purge gas-vaned anode type of HII system in which the problem of back corona is further reduced or even eliminated.
A further object is to provide an improved purge gas-vaned anode type of HII system in which back corona is at least further reduced as compared to the prior art, and at lower purge gas flow rates than heretofore practiced.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.